You may be telling yourself right now, "Linux is so hard! There are a lot of commands and even more command options! There is no way I can master all of these commands and commit them to memory." If this is what you think, believe me, you are smart. It's insane to remember all the Linux commands that exist, even the most experienced Linux administrator would never be able to remember all commands, not even Linus Torvalds himself!

So wait? If that's the case, what is the solution then? The answer lies in the beautiful world of Linux documentation. Linux is very well documented to the extent that it's hard to get lost in it. There are a variety of tools in Linux that help you in not just remembering the commands, but also in understanding how to use them.

Having met a lot of Linux professionals throughout my career, I noticed that the most skilled Linux administrators are not the ones who remember, but the ones who know how to make the most use of the Linux documentation. Ladies and gentlemen, I highly recommend you fasten your seatbelt and read this chapter carefully. I promise you that the fear in your heart will go away soon!

The four categories of linux commands

All Linux commands must fall into one of these following four categories:

1. An executable program: Which is usually written in the C programming language. The cp command is an example of an executable command.
2. An alias: Which is basically another name for a command (or a group of commands).
3. A shell builtin: The shell supports internal commands as well. The exit and cd commands are two examples of a shell builtin command.

4. A shell function: These are functions that help us achieve a specific task and are essential in writing shell scripts.  We will cover this in more detail later, for now, just know they exist.

Determining a command's type

You can use the type command to determine the type (category) of a command. For example, if you want to know the type of the pwd command you can simply run the type pwd command:

elliot@ubuntu-linux:~$ type pwd 
pwd is a shell builtin

So now you know that the pwd command is a shell builtin command. Now let's figure out the type of the ls command:

elliot@ubuntu-linux:~$ type ls
ls is aliased to `ls --color=auto'

As you can see, the ls command is aliased to ls --color=auto. Now you know why you see a colorful output every time you run the ls command. Let's see the type of the date command:

elliot@ubuntu-linux:~$ type date 
date is /bin/date

Any command that lives in /bin or /sbin is an executable program. Therefore, we can conclude that the date command is an executable program as it resides in /bin.

Finally, let's determine the type of the type command itself:

elliot@ubuntu-linux:~$ type type 
type is a shell builtin

It turns out the type command is a shell builtin command.

Finding a command's location

Every time you run an executable command, there a file somewhere on the system that gets executed. You can use the which command to determine the location of an executable command. For example, if you want to know the location of the rm command, you can run the which rm command:

elliot@ubuntu-linux:~$ which rm
/bin/rm

So now you know that rm lives in the /bin directory. Let's see the location of the reboot command:

elliot@ubuntu-linux:~$ which reboot
/sbin/reboot

As you can see, the reboot command lives in the /sbin directory.

What does the command do?

You can use the whatis command to get a brief description of what a command does. For example, if you want to know the purpose of the free command, you can run the whatis free command:

elliot@ubuntu-linux:~$ whatis free
free (1)             - Display amount of free and used memory in the system

As you can see, the free command, as we already know, displays the amount of free and used memory in the system. Cool! Now let's see what the df command does:

elliot@ubuntu-linux:~$ whatis df
df (1)               - report file system disk space usage

Finally, let's see what the which command does:

elliot@ubuntu-linux:~$ whatis which 
which (1)            - locate a command

As we already know, which displays a command's location.

The man page

The whatis command gives you a brief description of what a command does; however, it doesn't teach you how to use a command. For that, you can use the man pages.

The man page is a manual page that has proper documentation to help you understand how to use a command. The same as when you buy a new phone, you get a manual that shows you how to use your phone and how to update your software on your phone, etc.

In general, if you want to read the man page of a command, you can run:

man command_name

For example, if you want to view the man page of the touch command, you can run the man touch command:

elliot@ubuntu-linux:~$ man touch

As you can see in the preceding screenshot, the touch man page shows you how to use the command, and it also lists and explains all the command options.

Table 9 shows you how to move around while browsing man pages.

I can't stress enough the importance of man pages. They can be your best friend in the darkest moments, trust me!

You should also know that there is a man page for man itself:

elliot@ubuntu-linux:~$ man man

It describes how to use man pages.

Help for shell builtins

If you play around enough with man pages, you may notice that a lot of shell builtin commands do not have a man page. For instance, there is no man page for the cd or the exit commands:

elliot@ubuntu-linux:~$ type cd 
cd is a shell builtin 
elliot@ubuntu-linux:~$ man cd 
No manual entry for cd 
elliot@ubuntu-linux:~$ type exit 
exit is a shell builtin 
elliot@ubuntu-linux:~$ man exit 
No manual entry for exit

That's because shell builtin commands do not have man pages, but do not freak out just yet! You can still find help on how to use shell builtins by using the help command. For example, to get help on how to use the exit command, you can run:

elliot@ubuntu-linux:~$ help exit 
exit: exit [n]
    Exit the shell.

    Exits the shell with a status of N. If N is omitted, the exit status 
    is that of the last command executed.

Similarly, to get help on how to use the cd command, you can run the help cd command:

elliot@ubuntu-linux:~$ help cd 
cd: cd [-L|-P] [dir]
    Change the shell working directory.

    Change the current directory to DIR. The default DIR is the value of 
    the HOME shell variable.

    The variable CDPATH defines the search path for the directory containing DIR.  
    Alternative directory names in CDPATH are separated by a colon (:). 
    A null directory name is the same as the current directory. 
    If DIR begins with a slash (/), then CDPATH is not used.

    If the directory is not found, and the shell option `cdable_vars' is set, 
    the word is assumed to be a variable name. If that variable has a value, 
    its value is used for DIR.

    Options:
        -L force symbolic links to be followed
        -P use the physical directory structure without following symbolic links
    The default is to follow symbolic links, as if `-L' were specified. 

    Exit Status:
    Returns 0 if the directory is changed; non-zero otherwise.

The info page

The GNU project launched the info pages as an alternative documentation to the man pages. The GNU project once claimed that man pages are outdated and needed replacement and so they came up with the info pages.

You can view the info page of any command by running:

info command_name

For example, to view the info page of the ls command, you can run the info ls command:

elliot@ubuntu-linux:~$ info ls

Next: dir invocation, Up: Directory listing

10.1 ‘ls': List directory contents
==================================

The ‘ls' program lists information about files (of any type, including directories). Options and file arguments can be intermixed arbitrarily, as usual.

For non-option command-line arguments that are directories, by default ‘ls' lists the contents of directories, not recursively, and omitting files with names beginning with ‘.'. For other non-option arguments, by default ‘ls' lists just the file name. If no non-option argument is specified, ‘ls' operates on the current directory, acting as if it had been invoked with a single argument of ‘.'.

By default, the output is sorted alphabetically, according to the locale settings in effect.(1) If standard output is a terminal, the output is in columns (sorted vertically) and control characters are output as question marks; otherwise, the output is listed one per line and control characters are output as-is.

Because ‘ls' is such a fundamental program, it has accumulated many options over the years. They are described in the subsections below; within each section, options are listed alphabetically (ignoring case). The division of options into the subsections is not absolute, since some options affect more than one aspect of ‘ls''s operation.

The info pages sometimes offer more details compared to man pages. However, man pages remain the most popular go-to destination for help documentation on Linux.

The very helpful apropos command

The apropos command is one of the most helpful and yet underrated Linux commands. Let's see a brief description of what the apropos command does:

elliot@ubuntu-linux:~$ whatis apropos
apropos (1)          - search the manual page names and descriptions

WOW! The apropos command helps you in searching for the right command to use to achieve a specific task. For example, let's say you want to rename a file, but you are unsure which Linux command to use; in this case, you can run the apropos rename command:

elliot@ubuntu-linux:~$ apropos rename
file-rename (1p)     - renames multiple files
File::Rename (3pm)   - Perl extension for renaming multiple files 
gvfs-rename (1)      - (unknown subject)
mmove (1)            - move or rename an MSDOS file or subdirectory 
mren (1)             - rename an existing MSDOS file
mv (1)               - move (rename) files 
prename (1p)         - renames multiple files 
rename (1)           - renames multiple files
rename.ul (1)        - rename files

BOOM! It listed all the commands that have the word rename showing in the description of their man pages. I bet you can spot the mv command in the output.

Let's say you want to view the calendar but you're unsure which command to use; in this case, you can run:

elliot@ubuntu-linux:~$ apropos calendar
cal (1)              - displays a calendar and the date of Easter
calendar (1)         - reminder service
ncal (1)             - displays a calendar and the date of Easter

You can see that it displayed the cal command in the output.

For the last example, let's say you want to display your CPU information, but you don't know which command to use; in this case, you can run:

elliot@ubuntu-linux:~$ apropos cpu 
chcpu (8)            - configure CPUs
cpuid (4)            - x86 CPUID access device
cpuset (7)           - confine processes to processor and memory node subsets 
lscpu (1)            - display information about the CPU architecture
msr (4)              - x86 CPU MSR access device
sched (7)            - overview of CPU scheduling
taskset (1)          - set or retrieve a process's CPU affinity

Here you go! You can see that it listed the lscpu command that we have used earlier. The apropos command is here to rescue you whenever you forget a command or you're unsure which command to use. You just have to supply a keyword (preferably a verb) that highlights what you want to accomplish to the apropos command:

apropos keyword

elliot@ubuntu-linux:~$ man -k cpu 
chcpu (8)            - configure CPUs
cpuid (4)            - x86 CPUID access device
cpuset (7)           - confine processes to processor and memory node subsets 
lscpu (1)            - display information about the CPU architecture
msr (4)              - x86 CPU MSR access device
sched (7)            - overview of CPU scheduling
taskset (1)          - set or retrieve a process's CPU affinity

The /usr/share/doc directory

The /usr/share/doc directory is another excellent place to look for help in Linux. This directory has very intensive documentation; it doesn't just show you how to use a command; sometimes, it will even show the name and contact information of the authors who developed the command. Moreover, it may also include a TODO file that contains a list of unfinished tasks/features; contributors usually check the TODO files to help fix bugs and develop new features.

To demonstrate, let's go to the nano documentation directory:

elliot@ubuntu-linux:~$ cd /usr/share/doc/nano 
elliot@ubuntu-linux:/usr/share/doc/nano$ pwd
/usr/share/doc/nano

Now list the contents of the directory to see what's inside:

elliot@ubuntu-linux:/usr/share/doc/nano$ ls
AUTHORS               copyright faq.html        nano.html   README    TODO 
changelog.Debian.gz   examples  IMPROVEMENTS.gz NEWS.gz     THANKS.gz

Cool! You can view the AUTHORS file to see the team of developers who contributed to the nano editor program. You can also view the TODO file if you are eager to know if there is anything left for you to do! You can also check the README file for a general description of the nano editor. There is even a link faq.html that contains frequently asked questions.

As you saw in this chapter, Linux has a variety of helpful tools available at your disposal; so make sure you utilize them!

Knowledge check

For the following exercises, open up your Terminal and try to solve the following tasks:

1. You need to know if the echo command is a shell builtin or an executable program, which command would you run?
2. Display the location of the uptime command executable file.
3. Show a brief description of the mkdir command.
4. You forgot how to use the mv command, what are you going to do?
5. You forgot which command is used to display the calendar, what are you going to do?
6. The history command is a shell builtin and so it doesn't have a man page. You want to clear your history but don't know how. What are you going to do?

True or false

1. The command whereis is used to locate commands.
2. You can use man -p and apropos interchangeably.
3. You can use the whatis command to get a brief description of a command.
4. You can use the type command to determine if a command is an alias, shell builtin, or an executable program.